The present invention relates to a pre-expanded particle of non-crosslinked linear low density polyethylene suitable for foaming within a mold which is able to close but unable to seal by heating with a heating medium such as steam.
In general, molding of a foamed article having a desired shape from pre-expaned particles of polyethylene is carried out by filling a mold which is able to close but unable to seal with pre-expanded particles, and heating the pre-expanded particles within the mold with a steam through a slit or small bores provided on the mold, whereby air within cells of the pre-expanded particle is expanded to cause expansion of the pre-expanded particles and, simultaneously, the pre-expanded particles are molten. As a result of the expansion of the molten pre-expanded particles, the space between the pre-expanded particles is closed and the molten pre-expanded particles are pressed against each other to give a desired foamed article in which the pre-expanded particles are fused together almost without clearance.
According to the process for foaming within the mold, occurance of temperature difference between the surface portion and the inner portion in the mold cannot be avoided, since the steam is introduced from outside the mold. The temperature difference becomes large when the mold is large and a desired article is thick. Also, when producing a desired foamed article having a thin portion and a thick portion, an optimum processing temperature varies at each portion.
A polyethylene such as a high density polyethylene (hereinafter referred to as "HDPE") or a high pressure processed, low density polyethylene (hereinafter referred to as "LDPE") is, however, inherently a crystalline polymer and has, in general, a narrow melting temperature range because of its crystallity. Accordingly, when the popular polyethylene is used as a material of the pre-expanded particle, a processing temperature capable of expanding the pre-expanded polyethylene particle in an optimum viscoelasticity is limited into a narrow temperature range. This fact requires a serious temperature control in the foaming process. That is, when the pre-expanded particles is heated to a temperature higher than the optimum processing temperature range, the melt viscosity of the particle becomes low, and thus the expanded air in the cells almost passed though the cell walls to the outside without expanding the cells. This fact results in lack of fusion between the pre-expanded particles. Even if the pre-expanded particles are expanded, the expanded cells in the foamed article thus obtained can easily shrink because of lack of inner pressure within the cells, or the cells become into open cell structure, which the foamed article easily shrinks and is reduced in mechanical propeties. When the pre-expanded particles is heated at a temperature lower than the optinum peocessing temperature range, the particle cannot be sufficiently expanded to give a foamed article having insufficient fusion between the particles and having a bad appearance due to remaining spaces between the particles.
In order to eliminate the above-mentioned disadvantages of the polyethylene, the polyethylene is crosslinked by using an organic peroxide or iradiating a radioactive ray to make the optimum processing temperature range thereof wide. However, such modification requires an additional crosslinking step and installation, which reduces the productivity and requires much cost. Also, since it is difficult to reuse the crosslinked polyethylene, the waste disposal of the foamed article has a problem.
In addition, for preparing the crosslinked polyethylene resin, the high-pressure processed LDPE is widely used as a starting material because the LDPE can be easily and well crosslinked. A pre-expanded particle obtained from the LDPE is superior in pliability and bufferability or cushoning property. However, the pre-expanded particle of the LDPE is inferior in heat-resistance and wants for solidity. Therefore, there can be used only the pre-expanded particle which has a relatively low pre-expansion ratio.
An object of the present invention is to provide a pre-expanded particle of a non-crosslinked polyethylene which has a wide optimum processing temperature range in the expansion molding within the mold, and is able to give a foamed article having a high expansion ratio and superior in heat-resistance, mechanical strength, pliability and cushioning property.
These and other objects of the present invention will become apparent from the description hereinafter.